[Bug c++/98419] New: wrong code when destructor of local variable modifies returned object

[Bug c++/98419] New: wrong code when destructor of local variable modifies returned object

[leni536 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98419

            Bug ID: 98419
           Summary: wrong code when destructor of local variable modifies
                    returned object
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: leni536 at gmail dot com
  Target Milestone: ---

Version:
g++ (Compiler-Explorer-Build) 10.2.0

Command line options:
-std=c++17 -O2 -pedantic-errors


```
struct A {
    int i;
    A(int ** iptrptr): i(1) {
        *iptrptr = &i;
    }
};

struct B {
    int* iptr;
    B(): iptr(0) {}
    ~B() {
        *iptr = 2;
    }
};

A foo() {
    B b;
    return A(&b.iptr);
}
```

Observed behavior:
foo() returns an object with its `i` member having the value 1.

https://godbolt.org/z/Yhcjo9

Expected behavior:
foo() to return an object with its `i` member having the value 2.

The destruction of `b` is sequenced before the initialization of the returned
object. The constructor of A sets the int* contained in `b` to point within the
returned object (there is no temporary created with type A, C++17's mandatory
copy elision is assumed here). ~B() then sets the `i` member of the returned
object to 2.

Other observations:
With command line options `-std=c++17 -O0 -pedantic-errors -fsanitize=address`
~B() tramples the stack:

https://godbolt.org/z/M5ETa9

When A has a user-defined copy-constructor or destructor then I get the
expected behavior:

https://godbolt.org/z/osqbfz
https://godbolt.org/z/ozzPaf

Presumably when the returned type is trivially copyable then the copy isn't
elided, with the assumption that it's not observable. In C++17 the copy elision
is mandatory, and it is observable even for trivially copyable types, as shown
in this example.

[Bug c++/98419] wrong code when destructor of local variable modifies returned object

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98419

Martin Liška <marxin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |marxin at gcc dot gnu.org

--- Comment #1 from Martin Liška <marxin at gcc dot gnu.org> ---
It seems you invoke a use-after-scope:

g++ pr98419.C -fsanitize=address,undefined && ./a.out 
=================================================================
==31013==ERROR: AddressSanitizer: stack-use-after-scope on address
0x7fffffffdf30 at pc 0x000000401532 bp 0x7fffffffdec0 sp 0x7fffffffdeb8
WRITE of size 4 at 0x7fffffffdf30 thread T0
    #0 0x401531 in B::~B() (/home/marxin/Programming/testcases/a.out+0x401531)
    #1 0x401293 in foo() (/home/marxin/Programming/testcases/a.out+0x401293)
    #2 0x4012fb in main (/home/marxin/Programming/testcases/a.out+0x4012fb)
    #3 0x7ffff676e151 in __libc_start_main (/lib64/libc.so.6+0x28151)
    #4 0x4010fd in _start (/home/marxin/Programming/testcases/a.out+0x4010fd)

Address 0x7fffffffdf30 is located in stack of thread T0 at offset 48 in frame
    #0 0x4011b5 in foo() (/home/marxin/Programming/testcases/a.out+0x4011b5)

  This frame has 2 object(s):
    [48, 52) '<unknown>' <== Memory access at offset 48 is inside this variable
    [64, 72) 'b' (line 17)
HINT: this may be a false positive if your program uses some custom stack
unwind mechanism, swapcontext or vfork
      (longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-use-after-scope
(/home/marxin/Programming/testcases/a.out+0x401531) in B::~B()
Shadow bytes around the buggy address:
  0x10007fff7b90: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10007fff7ba0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10007fff7bb0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10007fff7bc0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10007fff7bd0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x10007fff7be0: f1 f1 f1 f1 f1 f1[f8]f2 00 f3 f3 f3 00 00 00 00
  0x10007fff7bf0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10007fff7c00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10007fff7c10: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10007fff7c20: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10007fff7c30: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
  Shadow gap:              cc
==31013==ABORTING

[Bug c++/98419] wrong code when destructor of local variable modifies returned object

[leni536 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98419

--- Comment #2 from Lénárd Szolnoki <leni536 at gmail dot com> ---
This is probably invalid, I wasn't aware of
http://eel.is/c++draft/class.temporary#3

> When an object of class type X is passed to or returned from a function, if X has at least one eligible copy or move constructor ([special]), each such constructor is trivial, and the destructor of X is either trivial or deleted, implementations are permitted to create a temporary object to hold the function parameter or result object.

[Bug c++/98419] wrong code when destructor of local variable modifies returned object

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98419

Martin Liška <marxin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #3 from Martin Liška <marxin at gcc dot gnu.org> ---
Closing as invalid then.